Iron-manganese modified biochar, preparation method thereof and application of iron-manganese modified biochar in Pb-polluted soil remediation

Abstract

The invention discloses iron and manganese modified biochar, a preparation method thereof and application of the iron and manganese modified biochar in Pb-polluted soil remediation, agricultural waste pyrolysis biochar and an iron and manganese salt solution are subjected to coprecipitation modification, and a composite material loaded with amorphous Fe-Mn oxide is prepared. The material can efficiently fix Pb, Cd, As and other heavy metals in soil, the effective state of the heavy metals after remediation is reduced by 70-90%, and the material is suitable for a complex soil environment with the pH value of 4.0-8.5. The method is low in cost, environmentally friendly and suitable for large-area farmland and industrial site remediation.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental functional materials and soil remediation, and particularly relates to a preparation method of iron-manganese oxide modified biochar and its application in the remediation of heavy metal (especially Pb, Cd, As, etc.) contaminated soil. Background Art

[0002] 1. Current situation of soil heavy metal pollution

[0003] Industrial emissions, mining activities and agricultural fertilization have led to the aggravation of soil heavy metal pollution globally. Among them, lead (Pb) is listed as a priority control pollutant due to its toxicity and accumulativeness.

[0004] 2. Defects of existing remediation technologies

[0005] Chemical stabilization method: Commonly used agents such as phosphates and lime, but it is easy to cause soil compaction or secondary pollution;

[0006] Biochar remediation: The fixation ability of raw biochar to Pb is limited (<50%), and it is significantly affected by soil pH;

[0007] Single modification technology: For example, patent CN201510023456.X uses iron-modified biochar, but does not utilize the redox characteristics of Mn, and has a poor effect on As / Pb composite pollution.

[0008] 3. Technical gap

[0009] There is an urgent need to develop a multifunctional remediation material with high adsorption capacity, wide pH adaptation range and long-term stability. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to solve the problems of low heavy metal fixation efficiency and single function of existing biochar, provide an iron-manganese co-modified biochar and its large-scale preparation method, and clarify its remediation mechanism in composite polluted soil.

[0011] The present invention is achieved through the following solutions: An iron-manganese modified biochar, using agricultural waste pyrolysis biochar as a carrier, with an Fe-Mn oxide complex loaded on the surface, where the molar ratio of Fe to Mn is 1:1 to 3:1, and the specific surface area ≥ 200 m 2 / g.

[0012] The Fe-Mn oxide is amorphous, and the total loading amount is 5-15% of the mass of the biochar.

[0013] A preparation method of iron-manganese modified biochar, including: preparing biochar from pyrolysis raw materials, co-impregnating with iron-manganese salt solution, alkaline aging, drying and crushing.

[0014] The iron-manganese salt is a mixed solution of FeCl3 and KMnO4, and the total metal ion concentration is 0.05 - 0.5 mol / L.

[0015] A method for repairing heavy metal contaminated soil with iron-manganese modified biochar, in which the biochar is incorporated into the soil at a mass ratio of 1 - 5%, and cured for 7 - 30 days.

[0016] The heavy metals include at least one of Pb, Cd, and As.

[0017] The beneficial effects of the present invention are as follows:

[0018] 1. The Pb / Cd fixation rate ≥ 90% (TCLP method), and the effective state of As is reduced by more than 70%;

[0019] 2. After repair, the proportion of the residual state of heavy metals in the soil is increased to 60 - 80% (BCR sequential extraction method);

[0020] 3. The material cost < 2000 yuan / ton, which is suitable for large-scale farmland restoration. Description of the Drawings

[0021] Figure 1 It is a graph of the content of available Pb in the soil half a month after applying iron-manganese modified biochar. Detailed Embodiments

[0022] The following is a further description of the present invention in conjunction with Figure 1 However, the protection scope of the present invention is not limited to the described content.

[0023] For clarity, not all features of the actual embodiments are described. In the following description, well-known functions and structures are not described in detail because they would obscure the present invention with unnecessary details. It should be considered that in the development of any actual embodiment, a large number of implementation details must be made to achieve the specific goals of the developer, such as changing from one embodiment to another according to the relevant system or relevant business restrictions. Additionally, it should be considered that such development work may be complex and time-consuming, but it is only routine work for those skilled in the art.

[0024] Example 1: Preparation of Iron-Manganese Modified Biochar

[0025] Step 1: Preparation of Biochar

[0026] Select corn straw as the raw material, dry it naturally and then crush it to a particle size ≤ 5 mm. Place the crushed straw in a tubular furnace, heat it to 500 °C at a heating rate of 10 °C / min under nitrogen protection, and keep it for 2 hours to complete pyrolysis. After pyrolysis, cool it naturally to room temperature to obtain the original biochar. The biochar is in black granular form, and its specific surface area is detected to be 320 m 2 / g, with an average pore size of 4.2 nm.

[0027] Step 2: Preparation of the iron-manganese modified solution

[0028] Weigh 27.03 g of FeCl3·6H2O and 7.90 g of KMnO4, dissolve them separately in 500 mL of deionized water, and after mixing, obtain an Fe-Mn mixed solution with a total metal ion concentration of 0.2 mol / L (Fe:Mn molar ratio is 2:1). Use a magnetic stirrer to stir for 30 minutes to ensure complete dissolution.

[0029] Step 3: Modification treatment of biochar

[0030] Immerse 50 g of the original biochar in the above-mentioned mixed solution, with a solid-liquid ratio of 1:10 (i.e., add 50 g of biochar to 500 mL of solution). Place the mixed system in a constant temperature oscillator and oscillate at 25 °C and 150 rpm for 6 hours to allow the biochar to fully adsorb Fe 3+ and MnO4 - . Subsequently, adjust the pH to 9.0 with 1 mol / L NaOH solution and continue to oscillate and age for 24 hours. After aging, wash with deionized water until the conductivity of the filtrate < 100 μS / cm to remove unbound metal ions. Finally, place the modified biochar in an oven and dry at 80 °C for 12 hours, and pass it through a 100-mesh sieve (particle size ≤ 0.15 mm) after grinding to obtain the finished product of iron-manganese modified biochar.

[0031] The characterization results are as follows:

[0032] SEM-EDS analysis: It shows that Fe and Mn elements are evenly distributed on the surface of the biochar without agglomeration;

[0033] XRD pattern: No obvious Fe or Mn crystal peaks are detected, indicating that the loaded oxides are mainly amorphous;

[0034] Specific surface area: The specific surface area of the modified biochar is 280 m 2 / g, slightly lower than that of the original biochar, but still maintaining a high porosity;

[0035] Fe-Mn loading: Determined by ICP-MS, the total loading of Fe and Mn is 9.8% of the mass of the biochar.

[0036] Example 2: Remediation application of Pb-contaminated soil

[0037] Experimental design

[0038] Select contaminated soil around a lead-zinc mine. After testing, its total Pb content is 1850 mg / kg, the available Pb (extracted by TCLP) is 620 mg / kg, the soil pH is 5.3, and the organic matter content is 2.1%.

[0039] Repair steps

[0040] Soil pretreatment: Air-dry the soil and sieve it through a 2-mm sieve to remove stones and plant residues;

[0041] Biochar addition: Add the iron-manganese modified biochar prepared in Example 1 at 3% of the soil mass (i.e., add 30 g of biochar per kilogram of soil), and mix well;

[0042] Moisture adjustment: Add deionized water to make the soil water content reach 25%, place it in a plastic basin and cover it with plastic wrap (to prevent water evaporation), and cure it at room temperature (25 ± 2 °C);

[0043] Sampling and detection: Sampling is carried out at 0, 7, 15, and 30 days of curing, and the available Pb content and soil pH changes are measured.

[0044] The repair effects are as follows:

[0045] Pb fixation rate: After 30 days of curing, the available Pb content extracted by TCLP decreased to 48 mg / kg, and the fixation rate reached 92.3%;

[0046] Speciation transformation: BCR sequential extraction showed that the proportion of residual Pb increased from 12% initially to 68%;

[0047] pH change: The soil pH increased from 5.3 to 6.7, tending to be neutral;

[0048] Plant verification: After planting ryegrass for 30 days, the Pb content in the above-ground part decreased by 89% compared with the non-repaired group.

[0049] Example 3: Remediation of Cd-As co-polluted soil

[0050] The characteristics of the polluted soil were taken from the farmland around a smelter, with a total Cd content of 8.5 mg / kg, a total As content of 325 mg / kg, an available Cd (extracted by DTPA) of 5.2 mg / kg, an available As (extracted by NaHCO3) of 102 mg / kg, and a soil pH of 7.8.

[0051] Remediation process

[0052] After sieving the soil, add iron-manganese modified biochar at a mass ratio of 5% (i.e., add 50 g per kilogram of soil);

[0053] After mixing evenly, adjust the water content to 20% and cure for 21 days;

[0054] Simultaneously set up a control group (without adding biochar) and a single-metal pollution group (only Cd or As pollution).

[0055] The results are analyzed as follows:

[0056] Cd fixation effect: The content of available Cd decreased to 0.6 mg / kg (a decrease of 88.5%), and the proportion of residual state reached 75%.

[0057] As transformation effect: The available As decreased to 28 mg / kg (a decrease of 72.5%), and the proportion of As(V) increased from 40% to 85%.

[0058] Synergistic mechanism: Fe-Mn oxides oxidize As(III) to As(V) and co-precipitate with Cd 2 + to form a Cd-Fe-As complex;

[0059] Soil microorganisms: The dehydrogenase activity of the soil after remediation increased by 35%, indicating that biochar improved the microbial environment.

[0060] Example 4: Remediation of acidic mine soil (enhanced application)

[0061] Special scenario

[0062] The soil of a pyrite waste land has a pH of 3.5 and contains Pb (2100 mg / kg), Cu (450 mg / kg), and Zn (680 mg / kg).

[0063] Optimization plan

[0064] Add 5% calcium dihydrogen phosphate (based on the mass of biochar) to the iron and manganese modified biochar to enhance the precipitation of heavy metals under acidic conditions;

[0065] Adopt a layered remediation technology: Mix 4% biochar into the surface soil of 0-20 cm, and inject a biochar suspension (10 g / L) into the deep soil of 20-40 cm.

[0066] The remediation effects are as follows:

[0067] After 90 days, the available states of Pb, Cu, and Zn decreased by 94%, 87%, and 91% respectively;

[0068] The soil pH increased from 3.5 to 5.2, and the sulfate content decreased by 60%;

[0069] The leaching toxicity of heavy metals (GB 5085.3-2007) is lower than the limit value.

[0070] Example 5: Field demonstration verification

[0071] Implementation location

[0072] A Cd-polluted paddy field in a certain province (with an area of 1 hectare, soil Cd content of 1.8 mg / kg, and pH = 6.0).

[0073] Operation process

[0074] Fifteen days before transplanting rice seedlings, 200 kg of iron and manganese modified biochar (equivalent to a 3% addition) was spread per mu and plowed and mixed evenly;

[0075] Conventional planting management was carried out, and the Cd content of the rice was detected after harvest.

[0076] The results are as follows:

[0077] The Cd content of the rice decreased from 0.65 mg / kg to 0.12 mg / kg (meeting the national standard of <0.2 mg / kg);

[0078] The available Cd in the soil decreased by 76%, and the remediation effect continued for three crops.

[0079] Although the technical solutions of the present invention have been described and listed in detail, it should be understood that for those skilled in the art, making modifications to the above embodiments or adopting equivalent alternative solutions are obvious to those skilled in the art. These modifications or improvements made without departing from the spirit of the present invention all fall within the scope of protection required by the present invention.

Claims

1. A biochar modified with iron and manganese, characterized in that: Using agricultural waste pyrolysis biochar as a carrier, an Fe-Mn oxide composite is loaded on the surface, where the molar ratio of Fe to Mn is 1:1 to 3:1, and the specific surface area is ≥200 m 2 / g.

2. The biochar according to claim 1, wherein: The Fe-Mn oxide is amorphous, and the total loading amount is 5-15% of the mass of the biochar.

3. A preparation method of the iron-manganese modified biochar according to claims 1-2, characterized in that It includes: Preparing biochar from pyrolysis raw materials, co-impregnating with iron and manganese salt solutions, alkaline aging, drying and crushing.

4. The method according to claim 3, characterized in that: The iron and manganese salt is a mixed solution of FeCl3 and KMnO4, and the total metal ion concentration is 0.05-0.5 mol / L.

5. A method for repairing heavy metal contaminated soil by using the iron and manganese modified biochar described in claims 1-2, characterized in that: Mix the biochar into the soil at a mass ratio of 1-5% and cure for 7-30 days.

6. The method according to claim 5, characterized in that: The heavy metals include at least one of Pb, Cd, and As.

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